1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a DC test function, and more particularly to a semiconductor integrated circuit able to test the DC characteristics without a deterioration of the AC characteristics.
2. Description of the Related Art
In general, a method of testing a large scale integrated circuit (LSI) device consists of inputting a test pattern or signal to an LSI device under test, comparing an output value from the LSI device to determine whether or not a function of the LSI device is faulty, and measuring analogue-values of an input/output voltage or current and the like.
A DC characteristic test and an AC characteristic test are used in the same way for a small scale integrated circuit (SSI) or a medium scale integrated circuit (MSI), based on the test method used for an LSI.
The DC characteristic test measures the voltage of external terminals, which are an input terminal and a power source terminal, of an LSI device or the DC current passing through the external terminals in a known DC testing method, and the AC characteristic test measures a propagation delay time between input/output terminals, a transition duration of the output waveform, a set-up time of the input waveform, a holding time, the minimum clock pulse width, and the maximum clock frequency or the like.
In the prior art large scale integrated circuit (LSI) device provided with a DC test circuit, a first test circuit and a second test circuit are arranged between an internal main circuit and an output buffer circuit, and the DC test of an LSI device is effected by utilizing these test circuits. But, in the conventional LSI device having the above structure, a deterioration of the AC characteristics occurs due to the delay caused by the DC test circuit.
One of the AC characteristics is a propagation delay time between an input terminal and an output terminal, and when a signal is applied to an input pin or terminal in synchronization with a predetermined clock signal, a delay of a signal output from an output pin can be measured by the AC test as a propagation delay time within an LSI device.
When the above-noted first test circuit and second test circuit are inserted, a signal is delayed when a signal is a passed through gates of the first and second test circuits; i.e., the signal delayed due to factors other than the essential function of the interior logic of the LSI device, and the AC characteristics are deteriorated. In these circumstances, a user may be given an impression of an essential AC characteristic of the LSI device as an unreliable electronic product, and thus further improvement is needed.